DE2018895A1 - Capacitance measuring circuit - Google Patents

Description

PATENTANWÄLTE * ν «ν ^ v DIPL.-ING. H. LEINWEBER dipl-ing. H. ZIMMERMANN

8 Munich 2, Rosental 7, 2nd AUI ₀ .

Valley addr. Ulnpat Munich

τβΐ ΐοη den

pru \

Unsar Zaichait

Lw / h

Nils Aage Juul Eilersen, Gjrfngehusvej 226-232,

DK 2950 Vedboek, Denmark

Capacitance measuring circuit

The invention relates to a capacitance measuring circuit and, more particularly, to a circuit for comparing and measuring small capacitances and small changes in capacitance. The capacitance elements in question are referred to below as capacitors, regardless of whether they are in the form of conventional capacitors for general circuit purposes or in the form of special constructions for measuring physical values, such as e.g. B-, bridges in a pipe, contractions and expansions of structural parts, etc. Where the capacitance values of two capacitors are to be compared , one of them is called the main capacitor and the other is called the reference capacitor, although in many cases these two names are freely interchangeable.

There are circuits for comparing the capacitance values of a main capacitor and a reference capacitor

009846/1338

known in which both capacitors are connected to the terminals of an oscillator or another Source of AC input voltage through input circuits are connected, aie include rectifier couplings with opposite line directions, and which are also connected to these connections by output circuits, which are a consumer include, which belongs to the output circuits of both capacitors in common. That way, will an output DC voltage component is generated at this consumer, which represents the difference in the capacitance values of the two capacitors, and it can be measured by means of a direct current measuring instrument be measured, which itself has the shape of the common consumer or part of this consumer, to determine this difference.

However, it has been found that the mentioned DC voltage component is also not only depends on the peak-to-peak value of the input voltage but also on the degree of symmetry of the input voltage in terms of the positive and negative peaks and the relative duration of the positive and negative half cycles. When the symmetry in relation to the peak values is established by means of a voltage divider becomes impossible to connect both of the Oscillator as well as one terminal of each capacitor, as is most desirable for practical purposes is, and there may still be lack of symmetry in terms of the relative duration of the positive and negative half cycles may be present.

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The influence of the input voltage on the measurement result can of course by an input voltage control device should be switched off, but if this should also eliminate errors resulting from the If there is a lack of symmetry, it will have to be quite complex and expensive.

The capacitance measuring circuit according to the invention is characterized in that a main capacitor and a reference capacitor, each connected in series with an auxiliary capacitor, between terminals are connected for supplying an alternating voltage, that both the main capacitor and the reference capacitor are each shunted with two voltage-limiting Rectifier couplings with opposite line directions are and that at least a pair of the voltage-limiting rectifier couplings, which are connected to the main capacitor and the reference capacitor and opposite line directions have, are completed by a common consumer.

As will be shown below, by using the circle according to the invention it becomes possible the influence of both the peak-to-peak value and any lack of symmetry in the input voltage im essential off.

In preferred embodiments of the invention, some of the rectifier couplings have Zener diodes that can belong to the circuits of the main capacitor and the reference capacitor together or not. ._

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When high accuracy is not essential and the main purpose is to change the
Estimate the capacitance value of the main capacitor,
the circle can be simplified by the
Reference capacitor and all rectifier couplings,
associated with it are omitted.

The invention is described below with reference to the drawing. Show in it:

1 shows a circle according to a first embodiment
the invention;

2 shows a circuit according to a second embodiment of the invention;

3 shows a circle according to a third embodiment of the invention

and
4 shows a circle according to a fourth embodiment of the invention.

In the first embodiment of the invention shown in Fig. 1 there is a main capacitor
C shunted with a first voltage-limiting rectifier coupling, which consists of a diode Dl, and a second voltage-limiting rectifier coupling, which consists of a diode D2, a Zener diode Zl and a consumer RG-CG. A reference capacitor C3 is shunted to a first voltage limiting rectifier coupling which is made up of
a diode D4, and a second voltage

—5—

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_ 5 -

limiting rectifier coupling consisting of a diode D3 »a Zener diode Z2 and the consumer RG-CG consists of the second rectifier coupling of the main capacitor C belongs »The main capacitor is with an auxiliary capacitor C2 between the connections O and UOSC of an oscillator or a other suitable AC voltage source connected in series. The reference capacitor C3 is connected to an auxiliary capacitor C4 between the terminals 0 and ÜOSC connected in series.

The Zener voltage of the Zener diodes Zl and Z2 are denoted by UZl and UZ2.

The alternating voltage UOSC, which has a positive peak value Ul, a negative peak value U2 unü has a frequency f is applied to the circuit and it is assumed that CG ^) C, C3, C2 and C4 and that the output voltage at the consumer RG-CG, U << C UZl and UZ2, and that the threshold voltage of the diodes Dl, D2, D3, D4 in the direction of the line in comparison with UZl and UZ2 is negligible.

When UOSC takes the value Ul, C is charged to the voltage UZl through C2, and the rest of the charge, which is transmitted by C2, arrives via D2 and Zl to and through RG-CG when C is the Zener voltage UZl has reached. C2 will assume the voltage Ul-UZl.

Similarly, C3 is charged to voltage 0 across C4 since it is shorted through D4. C4 becomes assume the tension Ul.

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When UOSC takes on the value -U2, C is charged to 0 via C2 because it is short-circuited by Dl. C2 becomes the Accept voltage -U2.

C3 is charged to -UZ2 via C4, and the rest of the charge transferred from C4 is in via D3 and Z2 and pass through RG-CG when C3 reaches Zener voltage -UZ2. C4 will assume the voltage -U2 + UZ2.

When UOSC changes from -U2 to Ul, C2 transfers charge C2 (Ul- UZl + U2) to C and RG-CG.

Since C takes up the charge C UZl, RG-CG will receive the charge C2 (Ul - UZl + U2) - C UZl, which is the current f £ c2 (Ul - UZl + U2) -C UZlJ corresponds.

When UOSC changes from Ul to -U2, C4 transfers charge C4 (-U2 + UZ2-U1) to C3 and RG-CG.

Since C3 takes the charge -C3 UZ2, RG-CG will receive the charge C4 (-U2 + UZ2-U1) + C3 UZ2, which is the current f (-U2 + UZ2-U1) + C3 UZ27 corresponds.

_j

If the time constant RG. CG is chosen as T = 3, then a DC voltage component U is on generated by the consumer RG-CG satisfies the following equation

(I) U = RG. f £ C2 (U1-UZ1 + U2) -C UZlJ + RG. f £ ~ C4 (-U2 + UZ2

-Ul) + C3 UZ2J.

If C2 = C4 and UZl = UZ2 = UZ, then the above Equation reduced to the following form

(II) U = RG f UZ (C3-C).

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It can thus be seen: 'i that the value of U, which can be measured by means of a conventional measuring instrument, depends not only on the peak-to-peak value of the input voltage but also on the degree of symmetry of the input voltage, since all terms of equation (I), which contain the positive and negative peak values Ul and -U2, counterbalance each other and the relative Duration of the positive and negative half cycles in the equation do not occur.

It will also be seen from the expression of U that the circuit has a high DC output voltage and forms a low output impedance if only f and UZ are selected correspondingly high.

A reinforcement of U is unnecessary in most cases.

It is a great advantage that C, C3, U and the AC voltage source have a common ground connection to have.

If UZ is chosen to be high, however, it can be difficult to achieve sufficient uniformity of the temperature coefficients of UZl and UZ2 over a wide temperature range, and if UZl Φ UZ2 then the 0 point of U is strongly shifted.

This can be avoided by forming the circle with a single Zener diode Z but with two separate diodes D5 and D6, as shown in the embodiment of FIG.

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BATH ORIGINAL

In the case of this circle, the same equations can be used as in the case of the circle according to FIG. 1. The special feature of the circle according to Fig. 2, however, is that that D5, D6 and Z are connected to each other in such a way that the charge transferred from C2 to RG-CG passes through D2, Z, D6, while the charge transferred from C4 to RG-CG will pass through D3, Z, D5. So they become RG-CG transferred charges pass through the same Zener diode, eliminating the problem of different temperature coefficients of UZl and UZ2 in the circle according to FIG. Is switched off.

The ability to make a circle as shown in Fig. 2 effective depends on the fact that a threshold voltage of 0.3-0.6 volts must be exceeded before a diode conducts in the direction of the line, on the other hand, the charge to be transferred from C2 to RG-CG would not reach RG-CG via D2, Z, D6 but instead would take their way via D2, Z, Dj5, D4 and thus bypass RG-CG as soon as the voltage U became somewhat positive.

Similarly, the charge to be transferred from C4 to RG-CG would not make its way to RG-CG via D3, Z, D5 but Take via D3, Z, D2, Dl and thus bypass RG-CG as soon as the voltage U at RG-CG becomes somewhat negative.

Due to the threshold voltage of the diodes, the charges will pass through the RG-CG as long as the sum of U + the threshold of D6 (in the case of U ^ O) is smaller is than the threshold voltage of D3 + D4. It is similar in the case of U <0.

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In practice the result will be such that U has U <C-0.25 Volts is limited, but since the output impedance can be very low, this simple measuring circuit can directly operate recording instruments so that expensive amplifiers are often unnecessary.

The measuring circuit according to FIG. 2, similar to that according to FIG. 1, is independent of the amplitude and the symmetry of UQSC, and C, C3, U and the AC voltage source have a common ground connection.

In the embodiment according to FIG. 3, C and C3 have a pair of common consumer circuits that work in the same way are arranged as in Fig. 2, but the simple diode connections Dl and D4 at C and C3 have been replaced through voltage-limiting rectifier couplings in the form of another pair of shared consumer circuits, which are symmetrical to the first pair of common consumer circuits and a Zener diode Z3 with a Zener voltage UZ3, diodes D7 and D8 and a consumer RGl-CGl have. In this way it becomes possible to get high values of the output voltage U of the circuit. It is assumed that CG and CGl ^ C, C3, C2, C4 and that the Output voltage U ^ UZ and UZ3 *

When UOSC takes the value Ul, C is charged to the voltage UZ through C2, and the rest that from C2 Transferred charge is via D2, Z, D6 to and through RG-CG arrive when C has reached the Zener voltage UZ. C2 will assume the voltage Ul-UZ.

Similarly, C3 is charged to the voltage UZ3 via C4, and the rest of the charge transferred by C4 will

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via D4, Z3, D7 to and through RGl-CGl when C3 has reached the Zener voltage UZ3. C4 becomes the tension Accept U1-UZ3.

When UOSC takes the value -U2, C is charged on -UZ3 via C2, and the rest of the charge transferred by C2 will get via Dl, Z3, D8 to and through RGl-CGl when C has reached the Zener voltage -UZ3. C2 becomes the tension -U2 + UZ3 accept.

C3 is charged to -UZ via.C4, and the rest of the charge transferred by C4 is to and through via D3, Z, D5 RG-CG arrive when C3 reaches the Zener voltage -UZ. C4 will assume the voltage -U2 + UZ.

When UOSC changes from -U2 to Ul, C2 transfers charge C (Ul - UZ + U2 - UZ3) to C and RG-CG.

Since C takes charge C (UZ + UZ3), RG-CG will take charge C2 (Ul-UZ + U2-UZ3) - C (UZ + UZ3), the current ff * C2 (IJl-UZ + U2 - UZ3) - C (UZ + UZ3)] is equivalent to.

When UOSC changes from Ul to -U2, C4 transfers charge C4 (-U2 + UZ - Ul + UZ3) to C3 and RG-CG.

Since C3 takes on the charge C3 (-UZ - UZ3), RG-CG will take up the charge C4 (-U2 + UZ - UL + UZ3) + C3 (UZ + UZ3), which is the current f £ c4 (-U2 + UZ - Ul + UZ3) + C3 (UZ + ^UZ 3)].

If the time constant RG. CG is chosen so that = j, then a DC voltage component U an

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the consumer RG - CG achieved with the following value

(III) U = RG. f £ c2 (ül - UZ + U2 - UZ3) - C (UZ + UZ3) J + RG. f [ C4 (-U2 + UZ - Ul + UZ3) + C3 (UZ + UZ3) J

If C2 = C4, then the above equation is reduced to the following form

(IV) U = RG. f (UZ + UZ3) (C3 - C).

Similarly, the voltage UA across RGl - CGI UA = RGl. f (UZ + UZ3) (C - C3).

It will be seen that when RG = RG1, the circuit has positive and negative output voltages will generate with the same numerical value, which is often advantageous. If only the initial value U is required, RGl - CGI can be selected with the value zero. It will be seen that the output voltage is not only from depends on the peak-to-peak value of the input voltage but also on the degree of symmetry of the input voltage is. With the circle according to FIG. 3, the disadvantage is that the zero point of U due to the temperature coefficient of Zener diodes can be displaced, as can occur in the circuit of FIG. 1, in the same way as in FIG Fig. 2 has been switched off, and also the influence of the temperature coefficients on the values U and UA can be switched off that the temperature coefficients UZ and UZ3 are chosen to be the same but with opposite signs.

It will be seen, however, that in the circuit of FIG. 3 the output voltage is as high as the output impedance can be made as low as desired by selecting f, UZ and UZ3 accordingly. aside from that C, C3 and U and the AC voltage source have in common Ground connections

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The circle shown in FIG. 4 corresponds to that of FIG. 1 with the modification that the reference capacitor C3 and all components connected to it, d. H. C4, D3, D4 and Z2 have been removed. The simplified circle 4 can be used in cases where measurement accuracy is not essential and where in particular the changes of the main capacitor are to be determined. The output voltage at RG-CG will be as follows: (V) U = RG. f [C2 (Ul - UZ + U2) - C. UZ].

It will be seen that if U1, U2 and UZ remain constant during a test period, the changes of U are proportional to the changes in C.

If desired, the output voltage can be measured with a reference voltage instead of directly which can be set to form a neutral state from which changes in C can be registered.

Although in all the embodiments shown, the consumer on which the to be used for measuring purposes Output voltage developed is shown in the form of a resistor RG and a capacitor CG connected in parallel If so desired, the load can also consist of some form of electrical impedance.

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Claims (6)

- 13 claims: (P Capacitance measuring circuit, characterized in that a main capacitor (C) and a reference capacitor (C3), each connected in series with an auxiliary capacitor (C2 or C4), between connections (O, UOSC) for supplying an alternating voltage are connected that both the main capacitor (C) and the reference capacitor (C3) each in a shunt with two voltage-limiting rectifier couplings with opposite one another Line directions are and that at least one pair of the voltage-limiting rectifier couplings that are connected to the Main capacitor (C) or the reference capacitor (C3) are connected and have opposite line directions, are completed by a common consumer (RG-CG or RGl-CGl). 2. capacitance measuring circuit according to claim 1, characterized in that a pair of voltage-limiting Rectifier couplings of the main capacitor and the reference capacitor that are not through the common consumer are completed, each consists of a rectifier diode, which the capacitor in question shorts in the direction of his line. 3. capacitance measuring circuit according to claim 1, characterized in that a pair of voltage-limiting Rectifier couplings of the main capacitor and the reference capacitor, which are through the common consumer are completed, each consisting of a Zener diode. -14- 9845/1338 ' 4. capacitance measuring circuit according to claim 1, characterized characterized in that a pair of voltage limiting rectifier couplings of the main capacitor and the Reference capacitor, which are completed by the common consumer, from a common Zener diode which is bridged by a series connection of two rectifier diodes, which have the same conduction direction, with the common point of the rectifier diodes is connected to the common consumer. 5. capacitance measuring circuit according to claim 4, characterized by two symmetrical pairs of voltage-limiting Rectifier couplings of the type characterized in claim 4, each of these pairs having a common consumer having. 6. capacitance measuring circuit according to claim 1, characterized in that the reference capacitor and with it connected rectifier couplings are omitted. 009845/1 338 Blank page